1. Field of the Invention
The present invention relates to a method of sensing motion in three-dimensional (3D) space, and more particularly, to a method of removing rotational influence when a device is moved, converting coordinate axes of the device into coordinate axes of gravity, and sensing the motion of the device in the 3D space.
2. Description of the Prior Art
When a device, such as a 3D mouse, is moved, a user may make slight rotations of the device unconsciously. Because a three-axis accelerometer installed in the device is very sensitive, values of the three-axis accelerometer may be composed of a gravity component and acceleration of a hand of the user. Due to the acceleration of the hand, using a traditional coordinate algorithm to calculate the values of the three-axis accelerometer may not generate a correct motion trajectory of the device in the 3D space. Due to an assumption that the values of the three-axis accelerometer are only influenced by gravity in the traditional coordinate algorithm, calculated values of the three-axis accelerometer relative to gravity generated through the traditional coordinate algorithm may have errors, so that the device cannot output a motion trajectory relative to gravity precisely.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a device 102 which projects a cursor trail on a screen 100 according to the prior art. As shown in FIG. 1, the device 102 moves and rotates to the left. Because the device 102 is unable to convert its motion trajectory from the coordinate axes of the device 102 into the coordinate axes of gravity, the cursor trail, which is projected on the screen 100 by the device 102, follows the rotation of the device 102 to change its direction of motion. That is to say, the cursor trail not only moves left but left and up.
“INITIAL SENSING INPUT APPARATUS” is disclosed in Taiwan Patent Publication No. 200639406. Regardless of whether an apparatus is static or moving, the apparatus and method disclosed in this prior art cannot convert coordinate axes of the apparatus into the coordinate axes of gravity. Therefore, a motion output in this prior art correspond to motion along the coordinate axes of the apparatus, not to displacement along the coordinate axes of gravity. That is to say, the motion output is changed corresponding to the relative coordinates between the apparatus and gravity.
“FREE SPACE POINTING DEVICES AND METHODS” is disclosed in Taiwan Patent Publication No. 200538751. A device and method disclosed in this prior art can only convert the coordinate axes of the device into the coordinate axes of gravity when the device is static or slowly moving. So, this prior art cannot convert the coordinate axes of the device into the coordinate axes of gravity when the device moves. That is to say, a motion output of the device is incorrect if the relative coordinates between the device and gravity change.
“Gyroscopic pointer and method” is disclosed in U.S. Pat. No. 5,898,421. The method in this prior art cannot calculate a relative relationship between an accelerometer and a gyroscope, so this prior art needs more than two gyroscopes to calculate information required for performing coordinate conversion.